The alveolar epithelium is comprised of two cell types, both of which are believed to be essential to life. Type II cells produce surfactant and function in alveolar repair. The thin cytoplasmic extensions of type I cells cover more than 95 percent of the alveolar surface, providing the short diffusion pathway critical for normal gas exchange. Despite the importance of establishing and maintaining a normal alveolar epithelium, little is known about the cellular and molecular factors which control alveolar epithelial phenotypic expression. In vivo, type II cells have the capacity to repair injured alveoli, acquiring at least some characteristics of the type I cell phenotype. From limited in vitro studies, it has been proposed that reversible transdifferentiation can occur between type I and type II cells. Albeit biologically intriguing and of importance to development and lung injury, this hypothesis has not been rigorously examined. The broad long-term objectives of the studies outlined in this proposal are to determine the extent of phenotypic plasticity (interconvertability) of type I and type II cells and to define the mechanisms that regulate their phenotypic expression. With the goal of performing cellular and molecular studies of alveolar epithelial phenotypic expression, the investigators have recently developed improved in vitro systems, additional markers for the type I and type II phenotypes, and methods of isolating highly purified type I and type II cells. Using these newly developed methods, they will test the underlying hypothesis that transdifferentiation can occur bi-directionally between type I and type II cells. With the markers for type I or type II phenotypes that are currently available, they will determine the extent to which transdifferentiation of alveolar epithelial phenotypes occurs and will identify cellular and molecular mechanisms regulating phenotypic expression in vitro. They will use highly purified preparations of cells both to study the functions of both types of cells and to identify new phenotype-specific genes. The experiments planned in this proposal should determine the extent to which transdifferentiation occurs, define specific cellular and molecular mechanisms responsible for regulating alveolar epithelial phenotypic expression, and delineate the first tested functions of type I cells.